The following is a brief description of the general types and classifications of positive-displacement pumps, the major components and operation of a positive-displacement pump (especially a plunger-type pump with reference to the examples shown in FIG. 1 and in FIG. 2 of the drawing), and prior art regarding the problems associated with maintaining the packing for the plungers.
A positive-displacement pump, sometimes referred to as a reciprocating fluid pump or as a reciprocating power pump, is a type of fluid pump driven by power from an outside source applied to the pump. There are several types of reciprocating power pumps. Typically, the pumps are classified as being plunger pumps or piston pumps. A plunger pump is differentiated from a piston pump in that a plunger moves past stationary packing, whereas a piston carries packing with it. A major problem associated with positive-displacement fluid pumps, especially high-pressure pumps, is that of providing a satisfactory seal for the piston or plunger. Another major problem is that the packing may initially provide adequate control of VOC emissions, but as the packing wears, the VOC emissions increase. Controlling VOC emissions requires frequent changing of the packing, which is expensive maintenance.
The pumps are also classified as either single acting or double acting. In a single-acting pump, liquid is discharged only during the forward stroke of the plunger or piston, that is, during one-half of the revolution. In a double-acting pump, liquid is discharged during both the forward and return strokes of the piston or pair of opposed plungers. That is, discharge takes place during the entire revolution.
Further, the pumps are often classified as being horizontal or vertical. In a horizontal pump, the axial centerline of the cylinder for the piston or plunger is horizontal. In a vertical pump, the axial centerline of the cylinder is vertical.
In addition, the pumps can be classified based on the number of plungers or pistons. A simplex pump contains only one piston or one plunger or a pair of opposed plungers driven by one connecting rod. A duplex pump contains two pistons or two plungers or two pair of opposed plungers driven by two connecting rods. A multiplex pump contains more than two pistons or two single-acting or opposed plungers. For example, a pump having three plungers or pairs of opposed plungers is commonly referred to as a triplex pump, and a pump having five plungers or pairs of opposed plungers is commonly referred to as a quintuplex pump.
Generally, a positive-displacement pump has a fluid end (sometimes referred to as the liquid end) and a power end.
The fluid end is that portion of the pump that handles the fluid. It consists of a pumping chamber (sometimes referred to as a compression, fluid, or liquid chamber or cylinder), and various ports, valves, and other components.
The pumping chamber is a chamber or plurality of chambers in which the motion of the plunger(s) or piston(s) is imparted to the liquid (or fluid). A piston or plunger is positioned to reciprocate in a cylindrical port, which can be considered to be the pumping chamber or a portion of the pumping chamber. The cylindrical port for the piston or plunger is a heavy-walled structure adapted for withstanding the high forces of containing the reciprocating piston or plunger.
A piston is a cylindrical body that is attachable to a rod and is capable of exerting pressure upon a liquid within the pumping chamber. A piston usually has grooves for containing rings that seal against the generally smooth interior cylindrical wall of the cylindrical port or against a replaceable cylinder liner placed in the cylindrical port as the piston reciprocates.
A plunger is a smooth rod that is attachable to a crosshead and is capable of exerting pressure upon a liquid within the pumping chamber. Sealing rings for a plunger are stationary, the plunger sliding within the rings. The cylindrical port for a plunger-type pump typically has two portions with different diameters, a plunger bore and an axially aligned packing bore. The packing bore has a larger diameter adapted than the plunger bore, so that the packing bore is adapted for accommodating packing between the interior cylindrical wall of the packing bore and the outward cylindrical surface of the plunger.
The pumping chamber can be made integral with a suction manifold and discharge manifold or can be made with separate manifolds. A suction manifold is a chamber that accepts liquid from the suction port(s) and distributes it to the suction valves. A discharge manifold is a chamber that accepts liquid from the individual discharge valves and directs it to the discharge port(s).
The power end is that portion of the pump in which the rotating motion of the crankshaft is converted to a reciprocating motion through connecting rods and crossheads. The power frame is that portion of the power end that contains the crankshaft, connecting rods, crosshead and bearings used to transmit power and motion to the fluid end.
The power frame of the power end is held in a substantially permanent, stationary position. The fluid end is typically bolted to the power frame and is cradled by the power frame. Sometimes, a frame extension connects the fluid end to the power frame when the fluid end is not bolted directly to the power frame. In any case, the fluid end is not unbolted and disconnected from the power end except for major maintenance overhaul of the fluid end.
The typical fluid end of a plunger-type pump includes a fluid-end pump body having at least one pumping chamber. The pumping chamber has a suction port (sometimes referred to as an intake port), a discharge port, and a cylindrical port (or, in the case of a double-acting plunger-type pump, a pair of opposed cylindrical ports). The cylindrical port in a plunger-type pump includes a plunger bore and an axially aligned packing bore. In some pumps, an internal lubrication port is provided for supplying lubricant to the packing bore, which lubricant can be distributed around an internal circumference of the packing bore by a lantern ring, as well know to those skilled in the art. An example of the fluid end of this type of pump with original packing and parts for the packing bore is illustrated in FIG. 1.
A suction valve is positioned in the suction port (e.g., in a cylindrical portion of the suction port that is sometimes referred to as the suction valve deck), and a discharge valve is positioned in the discharge port (e.g., in a cylindrical portion of the discharge port that is sometimes referred to as the discharge valve deck). In addition, a plunger is positioned to reciprocate in the cylindrical port having the packing bore and the plunger bore.
The suction valve is usually a spring-loaded check valve for allowing the flow of fluid from the low-pressure side of the pump through the suction port into the pumping chamber while preventing the backflow of fluid through the suction port. The discharge valve is usually a spring-loaded check valve for allowing the flow of fluid from the liquid cylinder through the discharge port to the high-pressure side of the pump with preventing backflow of fluid through the discharge port. Preferably, although not necessarily, the suction and discharge valves are vertically disposed in the pump, that is, the axis of each of the generally cylindrical valves is vertically oriented in the pump body. Furthermore, the vertical axes of the suction and discharge valves are preferably, although not necessarily, co-axially aligned.
The plunger of the pump is positioned to reciprocate back and forth in the cylindrical port of the pumping chamber. The cylindrical port consists of a heavy-walled structural body defining the plunger bore and the packing bore, of which at least the interior cylindrical volume of the plunger bore can be considered to be at least a portion of the pumping chamber. The heavy-walled cylinder of the cylindrical port is designed to withstand the high-reciprocating and high-pressure forces to accommodate the plunger. Typically, at the limit of its stroke, the plunger fills nearly the full length of the cylindrical port, and in some designs exceeds the full length of the cylindrical port and extends into another portion of the plumping chamber.
During the back stroke of the plunger, the withdrawal of the plunger increases the volume of the pumping chamber, which creates decreasing fluid pressure or suction in the chamber. This causes the suction valve in the suction port to open to draw fluid from the low-pressure side of the pump into the pumping chamber. The decreased fluid pressure in the chamber also causes the discharge valve in the discharge port to close, preventing fluid from the high-pressure side of the discharge port from backing up into the pumping chamber.
During the forward stroke of the plunger, the insertion of the plunger decreases the volume of the pumping chamber, which creates increasing fluid pressure in the chamber. This causes the discharge valve in the discharge port to open to pump fluid through the discharge valve to the high-pressure side of the pump. The increased fluid pressure in the chamber also causes the suction valve to close, preventing high-pressure fluid from the pumping chamber from being discharged through the suction port.
As mentioned above, a “packing bore” is provided adjacent the plunger bore in the cylindrical port. The packing bore has a generally cylindrical interior wall with an internal diameter (“I.D.”) that is larger than an internal diameter of the plunger bore and that is co-axially aligned with the plunger bore. An annular space is defined between the interior wall of the packing bore and a plunger extending through the packing bore into the plunger bore. In other words, the annular space is also substantially the same as the difference between the I.D. of the packing bore and the I.D. of the plunger bore.
The packing bore typically has a “seat” (sometimes referred to as a “land”) adjacent the high-pressure end thereof, which is toward the plunger bore. The seat is generally annular in shape, presenting an annular surface generally facing the low-pressure end of the packing bore, which is away from the plunger bore. The annular surface of the seat is preferably at a substantially perpendicular angle relative to the axis of the interior wall of the packing bore, but it can be at an oblique angle. The central opening in the seat allows for insertion of the plunger through seat. The seat of the packing bore can be formed as a shoulder between the interior wall of the packing bore and the plunger bore.
A removable “gland” (sometimes referred to as a “top gland” or “top piece”) is typically positioned adjacent the low-pressure end of the packing bore, which is away from the plunger bore. The gland is for axially capturing and squeezing the packing material or packing set positioned in the annular space within the interior wall of the packing bore against the seat of the packing bore. A central opening in the gland allows for insertion of the piston rod or plunger through the gland.
The gland is generally annular in shape, presenting an annular surface generally facing the high-pressure end of the packing bore, which is toward the plunger bore. The annular surface of the gland is preferably at a substantially perpendicular angle relative to the axis of the interior wall of the packing bore, but it can be at an oblique angle.
The removable gland typically is formed as a part of a body adapted to be removably secured to the body forming the interior wall of the packing bore. For example, the gland can have a circumferential flange or flange lobes through which bolts can be secured to the body forming the interior wall of the packing bore. In another design, the gland can have a circumferential threaded connector adapted to screw with a corresponding circumferential threaded connector on the body forming the interior wall of the packing bore, in which case the gland is sometimes referred to as a “gland nut.”
The packing bore is for accommodating relatively soft “packing” in the annular space between the interior wall of the packing bore and the plunger. The packing is for sealingly engaging the plunger to help prevent fluid leakage from around the plunger as it reciprocates in the plunger bore, which enables the compression of fluids in the pumping chamber.
The packing bore can accommodate various styles of packing. Historically, loose packing material was simply “stuffed” into the packing bore. Early on, packing material was formed into ring-shaped packing elements. The packing elements can be formed into rings having a rectangular or square cross-section. The packing rings can be split rings to facilitate assembly or removal of the packing rings from the packing bore. Because the packing material is relatively soft, a plurality of such packing elements is often backed-up with intermediate rigid washer-shaped rings or spacers. More recently, the engineering of the packing rings and other associated parts of the packing set has become increasingly sophisticated. The stack of the plurality of packing elements, intermediate spacers, and other pieces that can be used in the packing bore are collectively referred to as a “packing stack” or “packing set” or “packing assembly”.
The seat of the packing bore provides a land area for the packing set, including the packing and associated parts and pieces. With the packing rings and other pieces of a packing set are positioned in place in the packing bore against the seat, the plunger inserted through the packing set. Then the gland is then positioned in place over the packing set. The gland, when tightened, axially compresses and squeezes the packing set. This action causes the shape of soft packing material to distort, creating a tight sealing area between the packing bore and the outside diameter of the plunger, preventing any substantial leak of internal compressed fluids from around the plunger.
The packing material (or packing set) is axially captured and retained within the interior wall of the packing bore between the seat of the packing bore and the gland, which is positioned and tightened over the packing. Over-tightening of the gland on the packing can cause excessive friction as the plunger reciprocates through the packing elements, causing excess wear, heat, and even breakage of the plunger.
As mentioned above, a major problem associated with positive-displacement fluid pumps, especially high-pressure pumps, is that of providing a satisfactory seal for the plunger. This seal has normally been in the form of soft, nonabrasive packing elements adapted to seal the annular space between the pump plunger and the bore of the packing bore. During the power stroke of the plunger, the internal pump pressure acting axially on the packing set helps the packing rings to deform or deflect into sealing engagement between the reciprocating plunger and the packing bore.
Of course, the packing seals wear as the plunger reciprocates, and the fluid pumps require periodic maintenance to replace the worn seals. The wear on the plunger packing is a particularly serious problem when the fluid being pumped contains suspended particles of silt, clay, sand, or other abrasive material. The abrasive material tends to erode the packing causing early and frequent failure. Packing failure is normally evidenced by the leakage of fluid past the packing. A small amount of leakage can be tolerated, but when this becomes excessive, the pumping operation must be stopped to permit replacement of the packing.
The typical packing needs to be replaced every few months of pump operation. This maintenance involves tedious and time-consuming operations, including removal of the packing gland, removal of the worn packing elements from the packing bore, re-assembly of new packing elements in the packing bore, and replacement and proper tightening of the gland.
Eventually, typically after about two-to-three years of pump operation, however, the packing bore itself will require a major overhaul. During the reciprocating action of the plunger, the parts and pieces of the packing set have slight movement and this, along with corrosion, vibration and other factors, will cause the packing bore surface to deteriorate. Further, as the packing wears and loosens, the packing will in turn will increasingly wear on the interior cylindrical wall of the plunger bore. Eventually, the packing bore becomes useless as a sealing surface to prevent the compressed product from escaping from the pumping chamber to the pump exterior. Then it becomes necessary to recondition the packing bore diameters in a major overhaul of the pump. This is usually done by boring out the packing bore inside diameter to accommodate a sleeve, which replaces the original packing bore sealing surfaces with a new one.
Sometimes it is desirable to change the size of the plunger. The diameter of the packing bore, however, must be in a reasonable proportion to the diameter of the plunger and have a sufficient clearance to accommodate the cross-section of the packing. For example, a plunger having a 2-inch diameter can be positioned in a packing bore having 3-inch diameter, which provides a typical circumferential clearance of 0.5 inch. This allows for a packing material having a 0.5 inch cross-section (if square packing material is used) to fill the annular space between the outside diameter of the plunger and the internal packing bore diameter.
When it is desired to change the size of the plunger, the packing bore would then be of the wrong proportion. Many times, for example, it is desirable to increase pump internal pressures. One way of doing this is to decrease the plunger diameter. Doing this, of course, increases the clearance between the plunger bore and plunger outer diameter. Up to a reasonable extent, the increased clearance can be compensated with a packing having a larger cross-section. Alternatively, it is possible to re-bore and sleeve the original packing bore to reduce the internal diameter of the packing bore, and allow for the use of a packing having a more appropriate cross-section. However, this alternative requires major overhaul of the pump.
In many pumps, the packing bore is integrally formed as part of the fluid-end body. An example of this type of prior-art pump is illustrated in FIG. 1, which is hereinafter described in detail.
In a few pumps, a “stuffing box” is permanently captured in the fluid-end body by the attached power frame, in which case this stuffing box provides the packing bore. An example of this design is the Wheatley® “323” pump is illustrated in FIG. 2, which is hereinafter described in more detail. However, this stuffing-box design is adapted for major overhaul of the fluid end and does not allow for the removal of the stuffing box without removing the fluid end from the power frame. Essentially, the packing bore is formed in a non-integrally formed, but permanently installed stuffing box in a fluid-end body. The packing is routinely maintained without removal of this type of permanently installed stuffing box.
In other of pumps, a “stuffing box” is permanently bolted to the fluid-end body of the pump, although it can be removed without removal of the fluid end from the power frame. Such a separate stuffing box is massive and expensive because, in essence, it is a structural portion of the fluid end body. Essentially, the packing bore is formed in a non-integrally formed, but permanently attached stuffing box to a fluid-end body. The packing is routinely maintained without removal of this type of permanently attached stuffing box. When the packing bore wears to the point it needs major service, such a stuffing box portion of the fluid-end body can be removed for easier re-manufacturing or re-sleeving.